Substitution of amino acids during the manufacture of protein sequences is known to occur during protein translation or transcription. A mutation may occur in a DNA sequence, which causes the codon to change leading to the incorporation of the incorrect amino acid at that position during translation of the protein.
Furthermore, substitutions have been found to occur during protein synthesis caused by translation errors despite the fact that transcription correctly occurred. The translation error may result from the misreading of a (correct) DNA sequence or mischarging of a tRNA. This phenomenon has been reported in E. coli cells. More recently, the misincorporation of serine at asparagine positions in recombinant proteins expressed in Chinese hamster ovary (CHO) cells has been reported (Wen et al., Journal of Biological Chemistry, (2009) 284: 32686-94.)
Misincorporation can affect the ultimate folding and functionality of the protein that has the misincorporated amino acids present within its peptide sequence. Misincorporation can be determined by various methods known in the art, such as: intact measurement, peptide mapping analysis, or mass spectroscopy sequencing.
Wen et al. discovered that the substitution of serine at asparagine positions was due to the starvation of asparagine in a cell culture medium and that misincorporation could be limited by supplementing the medium with asparagine.